The present invention relates to an optical fiber laser that can be realized in lower cost, provided with higher reliability and enabled to be operated in high power.
Development of higher powered and lower cost light sources are required for the application in laser material processing and medical equipment. In order to meet this requirement, optical fiber lasers are enabling technology for generating highly efficient single-mode laser lights easily.
A conventional optical fiber laser uses such a double-clad type optical fiber 40 as shown in FIG. 4. The core region 41 of the double-clad type optical fiber 40 is doped by rare earth metals such as Nd, Yb, Er and Th. The clad region has a double structure in which an inner cladding region 42 having a refractive index lower than that of the core region 41 and an outer cladding region 43 having a refractive index lower than that of the inner cladding region 42 are included. The excitation light 44 propagates inside the inner clad region 42 in a multi-mode, and attenuates gradually as being absorbed in the center core region 41.
In case of high-powered optical fiber laser, the key technology is to introduce the excitation light into the inner clad region 42.
FIGS. 5A to 5C show major excitation methods in the conventional optical fiber lasers.
The method shown in FIG. 5A is called an end-face excitation method, in which the excitation light 53 generated by the semiconductor laser 52 oscillated in a multi-mode is introduced directly into the end-face of the optical fiber. There is another method for introducing the excitation light into the end-face of the optical fiber in which the excitation light generated by the semiconductor laser oscillated in a multi-mode is coupled to the multi-mode optical fiber at first, and then the coupled lights are bundled and focused, and finally coupled to the end-face of the inner clad region of the double-clad type optical fiber.
The method shown in FIG. 5B is called a V-groove method, in which the excitation lights 53 are focused into the V-groove 56 formed at the side face of the double-clad type optical fiber 51, and then the excitation lights are introduced into the inner clad region by way of reflection at the boundary surface.
The method shown in FIG. 5C is called a parallel-side excitation method, in which the multi-mode optical fiber 55 used for introducing the excitation lights is fused and connected onto the side face of the double-clad type optical fiber 51 (see U.S. Pat. No. 5,999,673). In this method, it will be appreciated that the intensity of the excitation lights can be increased by increasing the number of introducing ports, and that, the fluctuation of the laser power can be suppressed by using a plurality of semiconductor lasers, even if any of the semiconductor lasers may be failed.
FIG. 6 shows a general configuration of the optical fiber laser using the parallel-side excitation method. The excitation lights provided by the individual the semiconductor lasers 62, each configured in a single-emission structure, are introduced by the multi-mode optical fiber into the double-clad type optical fiber 61 in which the core region is doped by rare earth elements. The excitation light is introduced into the inner clad region of the double-clad type optical fiber 61 through the excitation light combiner 63 formed by fusing the double-clad type optical fiber 61 and the multi-mode optical fiber at the side-to-side position. In order to attain a high power, a plurality of semiconductor lasers 62 are connected as shown in FIG. 6. The fiber grating 64a that transmits the wavelength components corresponding to the wavelength of the excitation light and has a higher reflectance for the wavelength component corresponding to the wavelength of the oscillated light is formed at one end of the double-clad type optical fiber 61 (at the side into which the excitation light is introduced). In contrast, another fiber grating 64b reflecting partially the oscillated light is also formed at the opposite side to the place where the excitation light is introduced. Those two fiber gratings 64a and 64b operate as a total reflecting mirror and an output mirror for the laser resonator, respectively, and then, the laser oscillation light 65 is output. In order to provide a higher power, it is possible to introduce the similar excitation light at the opposite side where the laser light is emitted.